1. Field of the Invention
The present invention relates to a variable power optical system, consisting of a plurality of lens groups, for carrying out variable power by changing the distance between each lens group in the optical axis direction, an imaging apparatus including the variable power optical system, and a digital equipment mounted with the imaging apparatus.
2. Description of the Related Art
Recently, portable telephones and personal digital assistants (PDA) are being used widespread, and the specification of having a compact digital still camera unit or a digital video camera unit built into such equipment is becoming common. In such equipment, the imaging apparatus including a small imaging element having a low pixel number compared to a digital still camera and the like, and a single focal point optical system consisting of about one to three plastic lenses, which are independent products, is commonly being used due to strict restrictions on size and cost.
However, as the magnification of the single focal point optical system is about the same as visual observation, the object that can be photographed is limited to that near the photographer. In the present days where higher pixel and higher performance of the imaging element are rapidly advancing, a compact variable power optical system that is mountable to a portable telephone and the like, that handles high-pixel imaging element and that photographs even subjects distant from the photographer is becoming necessary.
For the variable power optical system of a compact configuration, a variable power optical system of three components of negative, positive, positive components consisting of a first lens group having a negative optical power, a second lens group having a positive optical power and a third lens group having a positive optical power, arranged in this order from the object side, is proposed in for example, U.S. Pat. No. 5,009,491 and U.S. Pat. No. 6,349,002. Further, in Japanese Patent Application Laid-Open No. 7-306362, a variable power optical system configured by five components including four components of negative, positive, positive, negative components in the order from the first lens group closest to the object is proposed in an aim to achieve a higher definition optical system. In these three prior arts, the first lens group is configured by a cemented lens, and thus the assembly adjustment is easy. Further, in Japanese Patent Application Laid-Open No. 7-306362, a configuration for shortening the entire optical length by appropriately setting the optical power of each lens group or displacement condition and the like of each lens group involved in variable power is disclosed.
However, the variable power optical system according to U.S. Pat. No. 5,009,491 has a great number of lenses of eight or nine lenses, and the entire optical length is also long. In the variable power optical system according to U.S. Pat. No. 6,349,002, the second lens group has large displacement amount, and as a result, the entire optical length becomes long, so that this optical system is not suitable for further compactness. The F-number of the wide-angle end is about 3.5 to 4 and is dark. The variable optical system of Japanese Patent Application Laid-Open No. 7-306362 has the first lens group configured with only the cemented lens and made into a simple configuration, but the total number of lenses is 10 or is a great number and thus the cost is high. Further, as a great number or five lens groups move when performing variable power, the device configuration including the driving device becomes complicated and as a result, the unit size becomes large.
Therefore, in the conventional variable power optical system, the number of lens increases and the entire optical length becomes long when having high-pixel imaging element. Thus, the conventional configuration is difficult to be made more compact.
As the error sensitivity within the lens group that carries out variable power becomes high when achieving compactness, the adjustment task between lenses is necessary. Consequently, in the conventional variable power optical system, simultaneous achievement of high-pixel imaging element, lower cost, and further a compact size that can be accommodated in the portable telephone or a personal digital assistant is not possible.